1. Field of the Invention
The invention relates to apparatus for use in a thermo-dynamic material testing system for optically measuring changes in specimen size that occur during material testing.
2. Description of the Prior Art
The materials industry extensively uses a variety of test methods and systems for measuring a wide range of different material characteristics. Since data about the performance and behavior of a particular material must generally be known before that material can be used in practice, material testing methods and systems have become critical to the materials industry. Such data is usually obtained by measuring the performance of a specimen of a particular material under controlled test conditions. For example, the specimen may be subjected to a increasingly large but controlled tensile force in order to determine the specific amount of tensile force that the specimen can withstand without fracturing or the tensile force at which the specimen will break apart. The manner through which the specimen reacts to applied test forces indicates how the underlying material will subsequently perform under actual operating conditions.
Generally speaking, a typical test specimen for use in performing tensile tests comprises a cylindrical length of material with a central portion having a reduced diameter. A longitudinal centrally located portion of this section is considered the gage length (or gage section) of the specimen. In a tensile test, changes in the length and cross-section of the gage length are measured to obtain information about the amount through which the material will "stretch" under load and the amount through which the cross-section of the material will "shrink" to accommodate the "stretch". For diametral measurements during tensile testing to have validity, these measurements are generally made at the center, i.e. the mid-span, of the gage length.
In conventional tensile testing, as the gage length of the specimen changes in size, the mid-span will also translate axially or lengthwise along the specimen. Most tensile testing machines known in the art are equipped with one stationary jaw and one movable jaw, which collectively transmit a tensile force to the specimen. The jaws grip the specimen at each of its ends and are then moved apart at a controlled rate and under a controlled force. Accordingly, as the jaws move apart to place the specimen under tension and thus cause its gage length to elongate, and specifically as the movable jaw travels away from the fixed jaw, the mid-span of the gage length translates along the longitudinal axis of the specimen and in same direction of motion as that of the moving jaw. When the change in length of the specimen is uniform around its gage length, the central portion of the gage length moves in the same direction of motion of the moving jaw but at a rate of travel which is one-half that of the moving jaw.
Machines designed to perform tensile and other similar tests have been in use for many decades. Many of these machines perform tensile tests at relatively slow rates. Traditionally, during such slow-rate testing, mechanical instruments have been used to physically measure the size of the gage length. In particular, to undertake these measurements, such an instrument was usually attached directly to the specimen in the area of the gage length and actually moved with the specimen during the tensile test. In recent years, material testing has also involved compression testing where the specimen is typically reduced in length by application of a controlled compressive force. Slow-rate compressive testing is usually performed with tensile-test type instruments that are attached directly to the specimen in the area of the gage length. In general, these instruments have exhibited adequate performance during such slow-rate testing.
However, during the last few decades, a need has also arisen to measure the mechanical size of the gage length at very high rates of specimen deformation. Unfortunately, mechanical instruments which are merely attached to the specimen gage length in order to measure its length and diameter have generally tended to be quite unreliable when subjected to high physical accelerations. Not surprisingly, in these situations, the instruments produced grossly erroneous readings. This effect stemmed from several causes such as, for example, additional mass and inertia which the instrument itself added to the specimen, difficulties associated with simply keeping the instrument adequately secured to a desired point on the specimen as both the instrument and that portion of specimen collectively moved at relatively high accelerations during high-speed specimen deformation, and the inability of the instrument itself to adequately perform when subjected to high accelerations.
Furthermore, material testing not only occurs at high rates but also at elevated specimen temperatures. In this regard, the temperature of a specimen may be very high during such tests which, in turn, causes additional difficulty in dynamically measuring its physical size while a test is underway. In particular, thermo-dynamic testing systems, particularly the "GLEEBLE" family of systems presently manufactured by the present assignee (which also owns the registered trademark "GLEEBLE"), self-resistively heat a metallic specimen by passing large controlled electrical currents through the specimen during testing in order to significantly raise its temperature. An example of such a system is described in my co-pending United States patent application entitled "A Test Specimen/Jaw Assembly that Exhibits Both Self-Resistive and Self-Inductive Heating in Response to an Alternating Electrical Current Flowing Therethrough", Ser. No. 07/645,190, filed Jan. 18, 1991 which has also been assigned to the present assignee hereof.
Now, to further complicate physical deformation measurement, even when a test specimen is only moderately hot, contacting the specimen with suitable measuring gauges is normally a difficult task at best. However, when the specimen temperature becomes extremely high, e.g. on the order of 3000 .degree. C. which can often occur during various tests, the task of physically contacting the specimen to make measurements becomes essentially impossible. Simply stated, such a gauge will itself physically expand and begin to distort, at much lower temperatures, i.e. well before these high temperatures are even reached, thereby totally frustrating the measurements.
Non-intrusive optical measuring techniques have been used widely in many manufacturing, testing and assembly applications to accurately locate the position of an object. In many such systems, changes in the position of an object are often determined by measuring changes in the size or shape of a light beam after that beam has been partially shadowed by the object. The light beam may be formed into a collimated sheet having a width greater than that of the object, or the beam may be a pencil beam that scans one- or two-dimensionally across the object. For example, an optical measuring system that uses laser beams shaped into collimated sheets of light for locating objects in a wind tunnel is described in "Electro-optical Position-Measuring Systems," NASA Tech Briefs. January 1991, page 26.
Thus, at first blush, in view of the need for a measurement technique that can function at very high deformation rates and at very high temperatures, optical measurement techniques, which rely on non-contact light based measurements and thus obviate the problems associated with traditional mechanical measurement techniques, would appear to hold great promise when used for measuring specimen deformation in a modern high speed thermo-dynamic material testing system. Accordingly, tensile and compressive testing machines are now frequently equipped with optical specimen measuring systems. However, while optical measurement systems are non-intrusive and yield very accurate results in many diverse applications, the optical measuring systems that to date have been applied to thermo-dynamic material testing systems have exhibited severe drawbacks which have greatly limited their usefulness.
In this regard, various non-intrusive optical techniques are known to measure specimen tensile deformation and deformation rates. Some of these techniques rely on measuring specimen gage length by allowing a light beam to pass through corresponding holes located at each end of the specimen or by illuminating flags that are spaced apart and positioned along the specimen. As noted above, some test specimens are prepared with a reduced mid-span that defines the gage length; this length being the portion situated between two effectively enlarged lateral walls. A light beam can be used to simply scan across the gage length, i.e. where a pin-point beam itself is oriented in a direction perpendicular to the longitudinal axial direction of gage length but is scanned along and in a direction parallel to the gage length, to detect changes in the spacing between the holes, flags or walls. Alternatively, a collimated light beam shaped as a sheet with a width greater than the gage length can be used, along with a suitable detector, to detect changes in the gage length, as defined by changes in the detected position of shadows cast by the locations of the individual holes, flags or walls as the specimen is being deformed. However, since the beam is scanned over a fixed one- or two-dimensional region of the specimen established prior to its deformation, these techniques fail to account for the axial translation of the mid-span of the gage length as the specimen deforms--thereby yielding inaccurate data.
Accordingly, some optical deformation measuring systems known in the art have tended to use a servo-controlled mirror to steer the light beam during specimen testing. An example of just such a system is the optical measurement system provided by Fuji Denpa Koki KK of Japan on its "THERMECMASTOR Z" induction heated thermo-dynamic testing machine. Other such systems move an optical transmitter or a receiver or both as the specimen changes size. To ensure that, for example, the mid-span is continually measured as it translates during specimen deformation, all of these particular optical measurement systems require tracking systems that, essentially, track the motion of the mid-span of the specimen. These tracking systems then attempt to move the beam in a pre-defined manner such that the beam continues to cross the mid-span as it translates longitudinally along the specimen. Unfortunately, these tracking systems tend to be quite complicated and expensive. Furthermore, and of much greater significance, these tracking systems, which include servo-controlled mechanical actuators to move either the mirror or the transmitter or receiver, generally have overall frequency responses that are often much less than that of the testing machines themselves, i.e. these testing machines can easily and in fact are routinely used to produce deformation rates in the specimen that greatly exceed the maximum rate at which, e.g., the mirror can be accurately positioned. Consequently, with such a tracking system and particularly at very high rates of deformation, the limited frequency response of the optical measurement system can introduce considerable phase lag into the specimen measurements. In fact, in some cases, this lag is so large as to effectively preclude optical measurements from being used at all at very high rates of deformation. In other cases, where such an optical measurement system is used, its response is likely to be simply too slow to follow the rapid translation in the mid-span, thereby producing measurements that simply contain excessive error and are thus of essentially no use.
One optical measurement system known in the art that does not rely on using a servo-controlled positioning system is described in U. S. Pat. No. 4,836,031 (issued Jun. 6, 1989 to Jatho et al and hereinafter referred to as the '031 Jatho et al patent). In particular, this patent describes a technique, which relies on an optical based measurement system, for performing fast-rupture tests on a test specimen. As described therein, separate mirrors are mounted on opposing jaws that grip opposite ends of the specimen and impart a tensile load thereto. A light source emits a collimated light beam that is then reflected by one mirror in a direction oriented along the longitudinal axis of the specimen. The beam travels along the length of the specimen and then strikes the other mirror which, in turn, reflects the beam into a position detector. The detector, which is spaced from the specimen, generates an output signal based upon positional translation (deviation) of the beam. The amount of deformation (denoted as .DELTA.s in the '031 Jatho et al patent) imparted to the specimen is determined based upon the measured deviation in the beam (there denoted as .DELTA.s') with simply the deformation rate being the rate of change in the measured deviation.
While the system described in the '031 Jatho et al patent appears to be able to measure an axial change in the length of the specimen, such a measurement is generally not sufficient to fully characterize the deformation which the specimen is undergoing. In this regard, such axial measurements do indicate the length through which the specimen will "stretch" during tensile testing but will not provide any indication of any change in the "shrinkage" of the cross-section of the specimen as it is being deformed in tension, let alone at the translating mid-span where such measurements should in fact be taken.
Thus, a need exists in the art for an optical based measurement technique, particularly one suited for use in a thermo-dynamic material testing system, for accurately and rapidly measuring changes in the cross-section of the translating mid-span of the specimen while the specimen is being deformed at a very high rate and at a very high temperature.